Colloidal semiconductor nanoparticles, named “quantum dots” (QDs), are crystalline objects that exhibit specific fluorescence properties. Their absorption cross section is very large, they are bright and their emission spectra have a small full width half maximum, and a peak wavelength that is tunable as a function of their composition, their size and their shape (in the range of a few nanometers to few tens of nanometers). They are also far more resistant to photobleaching than traditional organic dyes. These unique features make them very attractive for diverse applications in the field of medical and biological imaging, such as individual proteins monitoring, multi-color immunostaining, stem cells tracking, fluorescence acquisition cell sorting, or optically assisted surgery.
However, typical QD syntheses provide colloidal solutions of fluorescent nanocrystals capped with hydrophobic ligands, while the use of QDs in live-cell imaging requires their complete solubility in water as well as an excellent compatibility with biological media. To make the QDs water-soluble, two major methods exist: either the encapsulation of as-synthesized QDs with amphiphilic molecules into micelle-like structures (Dubertret et al. Science 2002, 298:1759), or a cap exchange, consisting in the replacement of original ligands by hydrophilic ones, bearing a chemical function able to bind to the nanocrystal surface (Chan et al. Science 1998, 281:2016 and Mattoussi et al. J. Am. Chem. Soc. 2000, 122:12142). Encapsulation is a very mild method, leading to the brighter nanoparticles, whereas cap exchange results in much smaller and more stable QDs. For both techniques, the non-specific interactions of the QD with cell membranes or with biomolecules in general depend mainly on the moieties that are adsorbed on the QD surface. In comparison to encapsulation, ligand exchange provides a versatile method to control the size, the nature of the ligand as well as its affinity for the QD surface (ligands that are too strong can indeed dissolve the QD, while ligands that are not strong enough can detach from the QD surface). This versatility explains why cap exchange is by far the most common technique to make the QDs water-soluble.
Ligand desorption is a strong limitation for the use of QDs in bio-imaging. This desorption, favored in high diluted conditions, causes indeed a loss of colloidal stability and functionality, as well as an increase in aggregation and non-specific adsorption. As a consequence, continued efforts have been made to improve the affinity of passivating ligands for the QD surface. The design of these replacing ligands is also guided by further needs for biological applications of quantum dots, namely: small size; stability over a large pH range, at elevated salt concentrations and in a cellular medium; low non-specific adsorption; and possible functionalization afterwards.
To match the above-mentioned criteria, the inventors first developed a zwitterionic ligand L1 connecting a dithiol, as bidentate linking function, to a sulfobetaine part, as aqueous solubility promoter (Scheme 1) (Muro et al. J. Am. Chem. Soc. 2010, 132:4556).

However QDs coated with this ligand L1 suffered from a lack of colloidal stability, mainly at high nanoparticle dilutions, due to ligand desorption.
There is thus a need for new ligands that exhibit high affinity for the QD surface (implying QD high colloidal stability), as well as a small size; a low non-specific adsorption; and possible functionalization afterwards.